For example, when a rectangular pulse signal representing a binary value, that is, either “0” or “1”, is output at a high speed from an output buffer of a Large Scale Integrated circuit (LSI) to a transmission path connected to the LSI, noise due to signal reflection or the like may arise when the pulse signal is input to a subsequent stage circuit. Such noise may cause a malfunction of the subsequent stage circuit. Thus, in order to make steep rising and falling of the waveform of the pulse signal more gradual, the adjustment of the slew rate for the pulse signal has been made. Here, the slew rate means the absolute value of a voltage change rate of a signal, and is represented by the value of a voltage that can rise per unit of time.
With respect to the slew rate for the pulse signal that is sent out to a transmission path at high speed, there is an appropriate range. For example, in the case where the slew rate is too small, the voltage change of a pulse signal becomes too slow, and as a result, a delay time until the voltage of the signal reaches a predetermined voltage increases. In the case where the slew rate is too large, the responsiveness to the voltage change is preferable, but, as described above, such a large slew rate may cause noise.
Thus, there is proposed a slew rate adjusting circuit configured to adjust a slew rate into an appropriate value that is empirically determined in accordance with the length of a transmission path and the state of signals output to the transmission path. For example, in Japanese Patent Application Laid-open No. 2006-245816 (JP2006-245816A), there is proposed a method that allows a slew rate to be adjusted using the difference between the periods of signals output from two different signal sources.
In the described-above slew rate adjustments in Background Art, however, there are disadvantages described below. In the case where, as in Japanese Patent Application Laid-open No. 2006-245816, the slew rate is adjusted using the difference between the periods of pulse signals output from two different signal sources, two signal sources are needed, and this causes increase in cost. Further, because of the existence of the signal sources that output signals having mutually different frequencies, the influence of asynchronous noise may increase.